In spodoptera frugiperda (Sf9) cells, both rTR-alpha and rTR-alpha-vI were localized in nuclei, were partly extractable by salt (0.4 M NaCl) and were phosphorylated. In band shift assay, Sf9-expressed rTRalpha formed two complexes with malic enzyme TRE, and both were readily recognized by the receptor specific antibody. Footprint analysis of these complexes showed an asymmetric binding of rTR-alpha to malic TRE which indicates that thyroid hormone receptor binds as a monomer and dimer to its target sequence. Co-expression of rTR-alpha and rTR-alpha-vI resulted in the decrease of only the upper complex. This observation suggests that there are two rTR- alpha species in Sf9 cells, only one of which can form homodimer or heterodimer.